Info

Sensor signal

Sensor signal

Fig. 2.5 A communications interface

2.6 Control of output devices

In many cases the commands from the computer are used to connect a circuit to earth, via a transistor. Two methods are used; one method is known as 'duty cycle' and the other is known as pulse width modulation. Each of these methods produces a different voltage pattern when the operation of the device is checked with an oscilloscope. The two patterns are shown in Fig. 2.6.

With duty cycle control the transistor that drives the device may be switched on for 100% of the time available, or for a small amount of the time available, for example 30%. The control of the device being operated is thus achieved by the length of the 'on time'. So a duty cycle reading of 50% means that the device, such as a petrol injector or mixture control valve, is switched on for 50% of the available time.

With pulse width control the transistor is switched on and off at fairly high frequency. The use of pulse width modulation (PWM) can reduce the heating effect in the solenoid of the device (injector) that is being operated.

Fig. 2.6(a) Duty cycle control of a mixture control solenoid

1 Current flow pulsed on and off enough to keep hold in winding activated.

2 Peak voltage caused by collapse of the injector coil, when current is reduced.

3 Return to battery (or source) voltage.

4 Injector ON-time.

5 Driver transistor turns on, pulling the injector pintle away from its seat, starting fuel flow.

6 Battery voltage (or source voltage) supplied to the injector.

Fig. 2.6(b) Pulse width modulation applied to a fuel injector

2.7 Computer memories

The term 'memory chip' derives from the fact that most computer memories are circuits that are made on a silicon chip. Automotive computers use memory chips that are very similar to those used in personal computers. The correct name for a 'chip' is integrated circuit or I/C. A large part of electronic memory is made of transistors and the number of transistors that can be made on a very small piece of silicon runs into millions.

Figure 2.7 shows a memory element known as a D type flip-flop. The D type flip-flop is called a memory device because it has the property that, whatever appears at D, either 0 or 1, will appear at Q when the clock pulse (C) is 1. When

Fig. 2.7 A computer memory element

C goes to 0, Q stops following the D value and holds the value it has when C changed to 0. Q will follow D again when C goes back to 1.

The logic gates shown in this diagram are made up from electronic components such as transistors, and large numbers of them can be made on a single integrated circuit.

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